CSS.supports undefined

[WebMechanic]

Hello,
I'm a bit confused...

It seems CSS.ts implements the CSS interface, however ...
the snippet below is some bit from a Vitest (0.34.6) test case with "@vitest-environment happy-dom" and @happy-dom/global-registrator set, loaded and enabled.

I need to test a script that makes calls to CSS.supports() which fails with "supports is not a function". I'm not interested in any results of CSS.supports() but the test suite won't execute load bc. of "supports" miraculously being undefined.

/**
 * @vitest-environment happy-dom
 */
test('CSS.supports', (ctx) => {
  expect(window).toBeDefined();       // pass
  expect(document).toBeDefined();     // pass
  expect(window.CSS).toBeDefined();   // pass
  expect(CSS).toBeDefined();          // pass

// throws "expected undefined not to be undefined"
  expect(CSS.supports).toBeDefined();
// throws "expected false to be truthy"
  expect('supports' in window.CSS).toBeTruthy();  
})

Why does window.CSS pass but CSS.supports is reported to be "undefined" and CSS.supports("fubar") fails with "supports is not a function"?

What am I missing here?
The script to be tested runs "flawless" in a real browser but has no test coverage because of this weirness.

Thanks 4 help.